Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It's 100% free, no registration required.

However, I see and hear this construction (んだから）all the time. It doesn't seem very rare at all, which leaves me confused as to why my native Japanese teachers told me that it was incorrect in the first place.

I'll use the first result from Google as the example.

お兄ちゃんのことがぜんぜんすきじゃないんだからねっ！！

This is a title for a movie. However, how would this be different if the title had have been:

お兄ちゃんのことがぜんぜんすきじゃないんだっ！！

If のだ is used as a way to indicate reason, then why is から also included? When would it be correct to use のだから instead of のだ?

4 Answers
4

Perhaps your teachers told you ～のだから (～んだから) is incorrect not because it is never used (you already know it's very common) but because you can't simply drop it into any sentence.

While digging around on Google, I came across a very nice PDF published by the Japan Foundation which explains the use of ～のだから. You can read it on your own (it's even got yomigana!), but I'll summarize the main points here.

Rules for ～のだから

Used when both the speaker and listener know some fact, but expresses a strong feeling on the part of the speaker that the listener, although conscious of said fact, does not fully appreciate its implications. (I have also seen this expressed as a mismatch between the speaker's and listener's perception of some matter.)

The clause following ～のだから often expresses the speaker's judgment, intent, wish, or request.

Example 1

A mother tells her children, 「10時半のバスに乗るから、早く支度しなさい。」 ("We're taking the 10:30 bus, so hurry up and get ready to go."). The children, however, are occupied with other things and don't do much in the way of getting ready. So the mother comes back and says, 「10時半のバスに乗るんだから、早く支度しなさい。」 ("[Hey, I know you know] we're taking the 10:30 bus, so hurry up and get ready to go."). ～から is used in the first sentence because it's new information and simply provides a reason for the latter clause. But ～のだから is used in the second because the children are aware of the fact that they're taking the 10:30 bus, yet their failure to get ready to leave shows that they don't fully appreciate its importance. ～のだから adds emphasis and serves to pull the listener's point of view around to the same side as the speaker's.

Example 2

A student comes up to a teacher and says, 「用事があるんですから、早めに帰りたいんですが。」 ("I have a prior obligation, so I'd like to leave early [if I could]…."). The teacher feels offended by this, because the use of ～のだから implies that the teacher should be aware of the fact that the student has something scheduled, when in fact the teacher has no such knowledge. (A better sentence would be 「用事がありますので、早めに帰りたいんですが。」 because ～ので does not presume that the listener already knows whatever precedes ～ので.)

There's a four-problem quiz in the linked PDF which is left as an exercise to the reader.

お兄ちゃんのことがぜんぜんすきじゃないんだからねっ！！

Context would help a bit here, but just as a guess, this is best filed under the "mismatch between perceptions" category of ～のだから: お兄ちゃん sees his 妹's actions as signs that she might actually like him, but in reality that's not the case, so 妹 decides to set the record straight and make sure she and お兄ちゃん are on the same page with ～のだから (the implication being that お兄ちゃん should be well aware of this fact, but from 妹's perspective, he doesn't seem to be fully appreciative of it).

Derek’s answer gives a very accurate description of what のだから means, but let me add a secondary point that is specific to your example: in this case, the title implies that in fact, the speaker likes her brother although the title states the opposite.

From Derek’s answer:

お兄ちゃん sees his 妹's actions as signs that she might actually like him, but in reality that's not the case, so 妹 decides to set the record straight and make sure she and お兄ちゃん are on the same page with ～のだから (the implication being that お兄ちゃん should be well aware of this fact, but from 妹's perspective, he doesn't seem to be fully appreciative of it).

All of this is from the sister’s perspective. But the very need of setting the record straight and making sure that she does not particularly like him is a sign that she actually likes him. Otherwise why would the brother think that his sister likes him in the first place?

Denying explicitly and strongly that the speaker likes the addressee while the speaker actually likes the addressee is such a stereotypical situation (probably mainly in comics) that there is a slang which describes the very situation: ツンデレ.

(By the way, the correct title is お兄ちゃんのことなんかぜんぜん好きじゃないんだからねっ!! This なんか means that the speaker does not think お兄ちゃんのこと as an important thing, which strengthens the speaker’s denial of the fact that she likes her brother, and again strengthens the actual implication that she actually likes him.)

お兄ちゃんのことがぜんぜんすきじゃないんだからねっ！！
This is a title for a movie. However, how would this be different if the title had have been:
お兄ちゃんのことがぜんぜんすきじゃないんだっ！！

1) That's because I really don't like him!
2) I really don't like him!

If のだ is used as a way to indicate reason, then why is から also included? When would it be correct to use のだから instead of のだ?

With のだ, you may say there is a reason. You may implicitly make a cause/consequence relation or a correlation. You were maybe not directly asked the reason to which you reply.
A: Doesn't he sing well?
B: I don't care, I don't like him anyway!

With のだから, you emphasise that THIS IS the reason.
A: Why don't you marry him?
B: Because I dislike him! Are you stupid or what?